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Full text of "Text Book Of Mechanical Engineering"

Appendix II.

09

the boiler; but it is dirty, and the stokehold temperature is as high
as n6d F. The closed ashpit is shewn by the Howden system,
Fig. 871, where the air is forced in at A by a fan, and passing
over tubes B B that are heated by hot gases from the boiler, enters
the fire through the fire-door c, both above and below the firebars.
The results are more economical than in the last system, but the
temperature of the stokehold is still high. In the third method,
shewn by the Meldrum furnace, Fig. 8710, air is introduced at the
blowers B, by means of a jet of steam from the pipe A, and the
ashpit is closed, but the ordinary firing arrangements are not
interfered with. The system is especially suitable for dust fuels.

Induced draught has many advocates. It is effected by a fan
in the uptake, which removes air from the boiler tubes and causes,
a partial v^fcuum into which the combustion air enters by passing
over the fire, Its best representative is the Ellis and Eaves system,
Fig. 872. The air enters by tubes A, where it is heated by furnace
gases, and then through the ashpit door as before. Upon reaching
the uptake it is deflected through the suction fan B, or may ascend
directly to the chimney if damper c be opened and D closed. Its
efficiency is greatly increased by the Serve tubes shewn, and
retarding plates, and the stokehold is both clean and cool. There
is said to be decreased injury to boiler tubes through the air
entering at their centres instead of impinging on their edges, and
an evaporation of 60 Ibs. per sq. ft. of grate is easily obtained.